patch 9.0.2168: Moving tabpages on :drop may cause an endless loop

Problem:  Moving tabpages on :drop may cause an endless loop
Solution: Disallow moving tabpages on :drop when cleaning up the arglist
          first

Moving tabpages during drop command may cause an endless loop

When executing a :tab drop command, Vim will close all windows not in
the argument list. This triggers various autocommands. If a user has
created an 'au Tabenter * :tabmove -' autocommand, this can cause Vim to
end up in an endless loop, when trying to iterate over all tabs (which
would trigger the tabmove autocommand, which will change the tpnext
pointer, etc).

So instead of blocking all autocommands before we actually try to edit
the given file, lets simply disallow to move tabpages around. Otherwise,
we may change the expected number of events triggered during a :drop
command, which users may rely on (there is actually a test, that expects
various TabLeave/TabEnter autocommands) and would therefore be a
backwards incompatible change.

Don't make this an error, as this could trigger several times during the
drop command, but silently ignore the :tabmove command in this case (and
it should in fact finally trigger successfully when loading the given
file in a new tab). So let's just be quiet here instead.

fixes:  #13676
closes: #13686

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2168,
 /**/
     2167,
 /**/